WaveMarket — which develops its own location-based mobile apps while also operating the Veriplace platform — previously landed a similar deal with Sprint, which brought its 50 million subscribers on board. Bringing AT&T into the fold, even if it’s just on a trial basis, boosts the number of mobile users in the Veriplace location network to 130 million, a substantial sample size.

The Veriplace platform gives third-party app makers access to a single API, including GPS data, to develop their apps and distribute them to subscribers across a variety of operating systems, including the iPhone’s OS, as well as handsets, says Tasso Roumeliotis, WaveMarket’s CEO.

Because Veriplace apps exist in the cloud, users don’t need to go to a site like the Apple App Store to download them. All they need to do is register with an application provider, give their permission to receive content, and the developers will send the apps directly to their phones.

“If you’re an enterprise and want to locate all your employees’ iPhones, for example — you can’t do that right now,” Roumeliotis says. “Now there will be an API for an app developer to locate iPhones from a server-side perspective.”

The two biggest demand drivers for location-based apps are advertising and social networking, he says. With Veriplace, apps can be rolled out to help social networking buddies find each other, regardless of whether they use Sprint or AT&T. Roumeliotis adds that other major carriers are expected to sign onto the initiative soon.

The Veriplace platform can also be used to serve advertising — and not just on the latest smartphones like the iPhone or Motorola Droid. As popular as these devices are, smartphones still account for only 25 to 30 million of the 250 million mobile devices in use in the U.S. And advertisers want to be able to reach as much as 70 percent of this base using legacy technology like SMS and text messaging.

Roumeliotis gives the example of Best Buy wanting to send an SMS message to consumers based on their current proximity to the nearest retail outlet.

“The Nexus One is one of many phones out there, but if you’re Best Buy and you have a common short code, that Nexus One phone is no different than a Motorola Razr circa 2004,” he explains.

The common API available via Veriplace emphasizes security, privacy and permission-based features. It is also sensitive to the concern some may have about telling others where they are at all times, Roumeliotis says.

WaveMarket’s best-known location-based app is called Family Finder, which helps parents keep track of their children using only their personal computers and GPS-enabled mobile devices. The application alerts parents when their child ventures beyond a preset “safe zone.” Family Finder is branded as AT&T FamilyMap, available through AT&T Mobility, and as Sprint Family Locator by Sprint Nextel. It has been rebranded by other carriers outside the U.S. too.

The success of Family Finder has made WaveMarket cash flow positive. It hasn’t had to raise money from its investors since 2005. At that time, it was backed by Draper Fisher Jurvetson, BlueRun Ventures, Qualcomm Ventures and Intel Capital.

So far, no company has devised a platform similar to Veriplace, allowing developers to create their own location-based mobile apps using a common API. When it comes to its own apps, however, WaveMarket competes indirectly with the likes of Loopt and Sniff, both of which help mobile users locate their friends on maps using GPS technology.